BACKGROUND: Treatment for uncomplicated stage 1 hypertension is recommended in most international guidelines but there is little evidence to indicate that therapy is beneficial. AIM: To estimate the prevalence of this condition in an untreated population and the potential costs of initiating therapy in such patients. DESIGN AND SETTING: Cross-sectional study of anonymised patient records in 19 general practices in the West Midlands, UK. METHOD: Data relating to patient demographics, existing cardiovascular disease (CVD), and risk factors (blood pressure and cholesterol) were extracted from patient records. Patients with a blood pressure of 140/90-159/99 mmHg, no CVD, and <20% 10-year cardiovascular risk were classified as having uncomplicated stage 1 hypertension. Missing data were imputed. The prevalence of untreated, uncomplicated stage 1 hypertension was estimated using descriptive statistics and extrapolated using national data. The cost of achieving blood pressure control in this population was examined in a cost-impact analysis using published costs from previous studies. RESULTS: Of the 34 975 patients (aged 40-74 years) in this study, untreated, uncomplicated stage 1 hypertension was present in 2867 individuals (8.2%, 95% confidence interval [CI] = 7.9 to 8.5). This is equivalent to 1 892 519 patients in England and Wales, for whom the additional cost of controlling blood pressure, according to guidelines, was estimated at £106-229 million per annum, depending on the health professional delivering care. CONCLUSION: Untreated, uncomplicated stage 1 hypertension is relatively common, affecting 1 in 12 patients aged 40-74 years in primary care. Current international guidelines and pay-for-performance targets, if followed, will incur significant costs for a patient benefit that is debatable.